The autonomic nervous system includes a sympathetic component and a parasympathetic (or vagal) component. The sympathetic component is relatively slow acting and is associated with a tendency to raise heart rate, blood pressure, and cardiac output. The parasympathetic component provides a relatively fast response and is associated with a tendency to reduce heart rate, blood pressure, and cardiac output. A proper balance between the sympathetic and parasympathetic components is a characteristic of cardio-neurological health. Variously referred to as autonomic balance, autonomic tone, sympathetic tone or sympathovagal balance, such a metric provides an indication of the patient's well-being.
Autonomic tone is affected by physical activity. In a clinical setting, autonomic tone is measured while the patient engages in a prescribed activity. For example, autonomic tone will increase while a user transitions from a supine position to an upright position. A typical test of autonomic balance, called an orthostatic test, does not adequately provide information as to tone over an extended period of time.